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Regions of positive slope in the pressure-discharge characteristics are one of the major concerns in design and operation of centrifugal pumps, as they are limiting the admissible operating range to values above the critical discharge. The industrial pump turbine of specific speed ν=0.42 (n q =66 min -1 ) proposed as QNET-CFD test case TA6-04 shows a marked saddle in the energy-discharge characteristic associated to a sudden drop of efficiency versus discharge at part load. The pump-turbine consists of a shrouded impeller with five blades, a diffuser with 22 guide and stay vanes and a spiral casing. CFD flow simulations on a reduced model were carried out with a finite volume Navier-Stokes code (CFX-5.7) using block-structured hexahedral meshes and the Menter-SST Turbulence model. Control of numerical quality has been performed. Reduced models with relatively low computational effort (mixing plane interface) already permit to capture the drop in efficiency and energy coefficient to analyze the flow phenomena inducing the drop of the energy coefficient Ψ that occurs at partial discharge.Analysis of local flow patterns and energy and velocity distributions at the rotor-stator interface that are related to the onset of recirculation are presented. INTRODUCTIONThe investigated industrial pump-turbine of specific speed ν=0.42 (n q =66 min -1 ) has been investigated experimentally by means of global performance measurements and local LDV measurements at guide vane inlet in pump mode and 2D-PIV measurements in the guide vane channels. They exposed the onset of a recirculating area at the hub side of the guide vanes that is related to the drop in the energy-discharge and the efficiency-discharge characteristics [1].

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View the article online for updates and enhancements. Abstract. Francis turbines may produce spontaneous pulsations of pressure and output power when operating at very high discharge. In such cases there is a cavitating central vortex in the draft tube with variable cavity volume V c . Until today, researchers agree that the main destabilizing agent is the so-called mass flow gain factor, defined as the derivative of cavity volume by the local discharge. Recent studies about 1D high-load stability analysis assumed that the mass-flow gain factor obtained from steady-state vortex data acts on the transient discharge downstream of the cavity. There are however good reasons to question this assumption. Most strikingly, the direct cause of the mass flow gain effect is the increase of swirl produced at the runner exit and hence upstream, not downstream of the cavity. To enhance the reliability of full-load stability predictions, the authors directly investigated the vortex dynamics. The development of the transient cavitating flow in the draft tube was simulated by means of unsteady 2-phase CFD. CFD work started with 1-phase calculations as presented by other authors. This was then extended to a more realistic 2-phase calculation. To contain the computing time within acceptable limits, given the very fine mesh and short time step required, the simulation domain was restricted to the draft tube and, at the same time, the problem was reduced to a basically 2-dimensional rotationally symmetric case. The response of the cavitating draft tube flow to a time-dependent inflow and time-dependent pressure at the draft tube exit was simulated. The results were input to a statistical identification procedure to check possible 1D transient models and find representative parameter values in the sense of a best fit between 1D model and CFD result. As we had suspected, the conventional vortex model with mass flow gain controlled by downstream discharge is not compatible with direct simulation and needs to be modified. The CFD results correspond to a model with the mass flow gain depending almost entirely on the runner exit discharge, delayed by a small dead time.